An integral part of an aircraft fuselage is the door assembly through which passengers and crew can enter and exit the aircraft. One type of door assembly that has become increasingly popular for use in commercial aircraft is the translating-motion-type door. This door, when opened, first moves up and away from the fuselage opening in which it is seated. Once the door clears the opening it is moved to one side of the opening so as to leave a free path for persons to enter and exit the aircraft. Translating doors take the place of conventional cocking type doors that are rotated inward and outward of the fuselage openings with which they are associated. A disadvantage of a cocking door is that, when opened, it is pivoted into the space inside the aircraft adjacent the entry/egress way adjacent the door. Consequently, the space inside the aircraft to the side of the door must be kept clear. A translating door, on the other hand, does not move into the space to the side of the door. As a result, when a translating door is installed in an aircraft, cabin units, such as galleys and lavatories, can be placed immediately adjacent the door entry/egress way. This increases the efficiency with which the space inside the fuselage is used. Also, translating-motion-type door assemblies weigh significantly less than cocking door assemblies for comparably sized openings. Moreover, translating doors typically include fewer components and are more economical to manufacture than similar cocking doors. Still another advantage of translating doors is that they move along a relatively straightforward up-and-out path when opened. In comparison to the inward rotation and outward movement of cocking doors, this feature makes translating-motion doors much easier to open. This feature is important because cabin attendants, who are often small in stature, sometimes find it physically difficult to open a cocking door.
An important part of any door assembly is the hinge that connects the door to the adjacent aircraft fuselage structure. A hinge used with a translating-motion-type aircraft door must be designed to allow the door to move up and down so that the door can move in and out of the associated fuselage opening. The hinge must further be arranged to allow the door to be moved away from the door opening so that the path in and out of the opening is clear. Moreover, once the door clears the opening, the hinge should allow the door to move along a path that is essentially parallel to the longitudinal axis of the airplane. In other words, the hinge is typically arranged so that, when the door is opened, the inside wall of the door is adjacent the outer skin of the aircraft. An advantage of this type of motion is that the effort required to move the door along a sliding path is substantially less than that required to pivot the door around the hinge as is required for cocking-type doors. Given that aircraft doors can weigh 300 pounds or more, any advances that reduce the effort required to open and close them are generally appreciated.
Many hinges used with translating-motion-type aircraft doors include both a structural hinge for securing the door to the aircraft and a mechanical linkage that provides a second door-to-fuselage connection. The mechanical linkage is employed to control the rotation of the door along its longitudinal axis so that, as the door is moved away from the fuselage, the door remains generally parallel to the outer skin of the aircraft. If the movement of the door was not controlled, the door would either swing free of the hinge and be difficult to maneuver and/or strike the outer skin of the fuselage.
A disadvantage of these hinges is that the linkages are composed of numerous interconnecting, movable mechanical parts. Providing these linkages appreciably adds to the overall cost of the hinges. Moreover, owing to the nature of their assembly, these mechanical linkages must be provided with rather long linkage members that add extra weight to the aircraft. This extra weight serves only to reduce the carrying capacity and operating efficiency of the aircraft. Owning to the nature of construction of these linkages, they are prone to require extensive and frequent maintenance. Furthermore, the rate at which these stiff linkage members apply angular momentum to rotate the door changes during the actual opening or closing cycle of door movement. As a result, as a door that is attached to this type of hinge is moved to the end of an opening or closing cycle, the amount of force needed to move the door increases. Moreover, the numerous cranks and drive rods of these linkages can be aesthetically displeasing to view.